gcc-6.3.1-2-bug.cpp

#include <list>

namespace ns {

struct S
{
    int i = 0;
    bool operator<(const S & other) const { return i < other.i; }
};

template<class vtx>
double distance(const vtx & v1, const vtx & v2) { return v2 - v1; }

void f()
{
  std::list<S> l;
  l.sort();
}

}

the errors

In file included from /usr/include/c++/6.3.1/list:64:0,
                 from gcc-bug.cpp:1:
/usr/include/c++/6.3.1/bits/list.tcc: In instantiation of ‘void std::__cxx11::list<_Tp, _Alloc>::merge(std::__cxx11::list<_Tp, _Alloc>&&) [with _Tp = ns::S; _Alloc = std::allocator<ns::S>]’:
/usr/include/c++/6.3.1/bits/stl_list.h:1617:14:   required from ‘void std::__cxx11::list<_Tp, _Alloc>::merge(std::__cxx11::list<_Tp, _Alloc>&) [with _Tp = ns::S; _Alloc = std::allocator<ns::S>]’
/usr/include/c++/6.3.1/bits/list.tcc:486:3:   required from ‘void std::__cxx11::list<_Tp, _Alloc>::sort() [with _Tp = ns::S; _Alloc = std::allocator<ns::S>]’
gcc-bug.cpp:17:10:   required from here
/usr/include/c++/6.3.1/bits/list.tcc:408:32: error: call of overloaded ‘distance(std::__cxx11::list<ns::S>::iterator&, std::__cxx11::list<ns::S>::iterator&)’ is ambiguous
        size_t __dist = distance(__first2, __last2);
                        ~~~~~~~~^~~~~~~~~~~~~~~~~~~
In file included from /usr/include/c++/6.3.1/bits/stl_algobase.h:66:0,
                 from /usr/include/c++/6.3.1/list:60,
                 from gcc-bug.cpp:1:
/usr/include/c++/6.3.1/bits/stl_iterator_base_funcs.h:135:5: note: candidate: typename std::iterator_traits<_Iterator>::difference_type std::distance(_InputIterator, _InputIterator) [with _InputIterator = std::_List_iterator<ns::S>; typename std::iterator_traits<_Iterator>::difference_type = long int]
     distance(_InputIterator __first, _InputIterator __last)
     ^~~~~~~~
gcc-bug.cpp:12:8: note: candidate: double ns::distance(const vtx&, const vtx&) [with vtx = std::_List_iterator<ns::S>]
 double distance(const vtx & v1, const vtx & v2) { return v2 - v1; }
        ^~~~~~~~